The present invention relates to lithography, and more particularly to a method of overcoming image shortening in a photolithographic process wherein sub-resolution reticle features are used to alter the aerial image in the shortened regions.
In order to increase the packaging density of various semiconductor devices formed on integrated circuits (ICs), the minimum feature size of the ICs must be reduced. This reduction in minimum feature size causes difficulties in various semiconductor device fabrication processing steps. One such area within the semiconductor device fabrication process which experiences unique challenges as feature size of the IC shrinks is photolithography.
Photolithography is used in the field of IC manufacturing to transfer patterns, i.e., images, from a mask (often called a reticle) containing circuit-design information to thin films on the surface of a Si wafer. The pattern transfer is accomplished with a photoresist (typically an ultraviolet light-sensitive organic polymer). In a typical image transfer process, a Si-wafer coated with a photoresist is illuminated through a mask and the mask pattern is transferred to the photoresist by chemical developers. Further pattern transfer into the underlying substrate is accomplished by using a chemical etchant.
Microphotolithography is employed in the semiconductor industry to form very small patterns (on the order of 0.25 xcexcm or less) on the surface of a substrate. Because of the limited resolution of current photolithographic tools, i.e., steppers, it is difficult to print dense array patterns in which the length of the individual lines is the same as that on the mask. Oftentimes when dense array patterns are printed, the printed image contains lines that are much shorter than those on the original mask. This problem is known in the art as xe2x80x9cimage shorteningxe2x80x9d.
The problem of image shortening of dense array patterns such as shown in FIG. 1A will now be described in more detail. Specifically, FIG. 1A shows a desired dense array pattern 10 that is formed on a reticle (not shown). The illustrated pattern of FIG. 1A includes rectangular clear features 12. The pattern illustrated in FIG. 1A is critical in semiconductor manufacturing, and is used in many different technologies such as in the fabrication of a capacitor level in trench-based dynamic random access memory (DRAM) cells, electrode patterns in stacked capacitor DRAMs, and contact arrangements in both DRAMs and microprocessors.
The issue of line shortening is especially important in the above-mentioned applications, where the length of the printed image is critical. However, fundamental-image shortening leads to undesirably short features 12xe2x80x2 as schematically shown in FIG. 1B. In this example, the exposure energy is chosen such that the feature width is imaged at the proper dimension. When this is the case, the feature length is much shorter than desired.
The most typical way of dealing with image shortening in dense arrays in prior art processes is by simple mask bias. In this case, the edges of the features on the reticle, i.e., mask, are extended as much as necessary to compensate the image shortening that occurs at the wafer level. As feature size shrinks and packaging density grows, this prior art method will fail because there is simply not enough room between shapes to move edges sufficiently.
Another prior art method to reduce image shortening is to add serifs to the mask features. Serifs are additional features added to the shape ends in order to provide aerial image tailoring. FIGS. 2A-2B are representations of mask serifs for unit cells of two different patterns. The added serifs are labeled as 16, whereas the pattern is labeled as 10. It should be noted that FIG. 2A represents a dark-field mask in which the rectangular features 12 and the serifs are clear features on an opaque background 14. In FIG. 2B, a bright-field mask is shown wherein features 12 and serifs 16 are opaque features on a clear background 14.
The serifs either add or subtract light in the areas where line shortening occurs, tailoring the aerial image in such a way as to compensate the shortening. There are many drawbacks to serifing, however. For example, the additional shapes are very small, making mask inspection and writing difficult, and each shape has multiple serifs added thereto so data volume for the mask gets very large. Furthermore, the efficacy of the added serifs decrease greatly as feature size decreases.
Lastly, it is known in the art that image shortening can be helped somewhat by changing the illumination conditions used to expose the mask. However, the conditions for minimized line shortening are rarely the same as the ones for best resolution, so this is typically not a viable option.
It is emphasized that image shortening is a quite different problem than proximity effects which is the problem of different lines imaging differently depending on their local environment. That is, proximity effect is an optical effect where diffraction causes features to interact with their neighboring feature. This interaction causes the image to print differently in one-dimension, i.e., wrong line width, lower process latitude, etc. The problem of image shortening is not. one-dimensional, rather it is a two-dimensional problem; therefore solutions that work in solving proximity effects do not necessarily work for reducing image shortening.
In view of the drawbacks mentioned hereinabove with prior art methods of reducing image shortening, there is a continued need to develop new and improved methods in which the effect of image shortening in photolithographic processes can be substantially eliminated, thereby printing an image on a semiconductor wafer that has substantially the same length as that of the pattern on the mask.
One object of the present invention is to provide a method which substantially reduces the image shortening problem typically observed when dense array patterns are printed.
Another object of the present invention is to provide a method of substantially reducing image shortening which is very easy to implement on a mask.
A further object of the present invention is to provide a method of substantially reducing image shortening which is easily inspectible.
A yet further object of the present invention is to provide a method of substantially reducing image shortening wherein no complex rules need to be generated, as is practiced with traditional serif and leveling bar technology.
These and other objects and advantages can be achieved in the present invention by placing sub-resolution reticle features perpendicular to the feature on the mask of interest. The term xe2x80x9csub-resolutionxe2x80x9d as used herein denotes an added feature which does not print on the substrate.
Specifically, the method-of the present invention which addresses the problem of image shortening in dense array patterns comprises the steps of:
(a) providing a mask having a dense array pattern formed thereon; and
(b) adding at least one sub-resolution reticle feature perpendicular to at least one feature of said dense array pattern, wherein each of said sub-resolution reticle features have a width smaller than the at least one feature of said dense array pattern.
In one embodiment of the present invention, the features of the dense array pattern and the added sub-resolution reticle features are clear features formed on an opaque background. Alternatively, the features of the dense array pattern and the sub-resolution reticle features are opaque features formed on a clear background.
In another aspect of the present invention, an improved photomask is provided. Specifically, the improved photomask of the present invention comprises:
a dense array pattern formed on a surface of a mask and at least one sub-resolution reticle feature located perpendicular to at least one feature of said dense array pattern, wherein each sub-resolution reticle feature has a width smaller than the at least one feature of said dense array pattern.
In this aspect of the present invention, the features of the dense array pattern and the added sub-resolution reticle features are clear features formed on an opaque background. Alternatively, the features of the dense array pattern and the sub-resolution reticle features are opaque features formed on a clear background.